Thermal interface adhesion for transfer molded electronic components

ABSTRACT

An electronic component to be encapsulated is introduced into a mold cavity. The mold cavity includes at least first and second halves, and at least one of the halves is formed with a negative of a thermal-interface-material engaging pattern thereon. An encapsulating material, which encapsulates the electronic component and engages the negative of the thermal-interface-material engaging pattern, is introduced into the mold cavity. The encapsulating material is allowed to solidify such that a thermal-interface-material engaging surface of the encapsulant solidifies with the thermal-interface-material engaging pattern thereon. During subsequent assembly, the thermal-interface-material engaging pattern engages thermal interface material to resist lateral motion of the thermal interface material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 15/060,137 filed Mar. 3, 2016, entitled “THERMAL INTERFACE ADHESIONFOR TRANSFER MOLDED ELECTRONIC COMPONENTS,” the complete disclosure ofwhich is expressly incorporated herein by reference in its entirety forall purposes.

STATEMENT OF GOVERNMENT RIGHTS

Not Applicable.

FIELD OF THE INVENTION

The present invention relates to the electrical, electronic, computer,and mechanical arts, and, more particularly, to electronic packagingtechnology, thermal control, and the like.

BACKGROUND OF THE INVENTION

Electronic components are typically encapsulated by transfer molding.For rapid and easy release from mold cavities, mold release agents areincluded in the formulation of transfer molding compounds. After themolding process, these release agents are present on the surface of themolded components. It is very challenging for thermal interfacematerials (TIMs) to adhere to the resulting very low surface energysurfaces (<35 ergs/cm²). In the past, the surface of the component hasbeen treated to provide higher energy surfaces for improved TIMadhesion. Such treatments include wiping the surface with isopropylalcohol, abrading the surface and then wiping with isopropyl alcohol, orexposing the surface to a plasma cleaning process or ultraviolet (UV)light and/or ozone. These are non-routine processes that are typicallyperformed at the card assembly factory and are significantly manual innature.

Heat dissipation has become a major technical challenge for complexprinted circuit board (PCB) assemblies. Rather than using individualheat sinks for every component, very large (in excess of four squarefeet or 0.37 square meters), common heat spreaders are being used overan array of components and component types. Due to the mismatch incoefficient of thermal expansion between the heat spreader, which istypically aluminum (coefficient of thermal expansion 23.6 ppm/C (partsper million per degree Celsius)) and PCB assembly (coefficient ofthermal expansion ˜17 ppm/C) and the large thermal mass of both the heatspreader and PCB assembly, significant relative movement occurs duringtemperature excursions resulting from powering the circuitry on and off.The TIM which physically bridges the gap between the heat spreader andPCB must accommodate this movement without breaking down. Because of theinherent low surface energy of transfer molded component surfaces, theTIM can slide along these surfaces when it is sheared or compressed bythe relative movement, and over time, can move out of the gap. Theresult is increasing thermal resistance which leads to decreasedreliability, and in a worst case scenario, fire. Because of the size ofthe PCB assemblies, batch processing in plasma or UV and/or ozone isvery costly. Abrasion and isopropyl wipe techniques are undesirable aswell because of the handling and debris generation.

SUMMARY OF THE INVENTION

Principles of the invention provide techniques for thermal interfaceadhesion for transfer molded electronic components. In one aspect, anexemplary method includes the step of introducing into a mold cavity anelectronic component to be encapsulated. The mold cavity includes atleast first and second halves, and at least one of the halves is formedwith a negative of a thermal-interface-material engaging patternthereon. Further steps include introducing into the mold cavity anencapsulating material which encapsulates the electronic component andengages the negative of the thermal-interface-material engaging pattern,and allowing the encapsulating material to solidify such that athermal-interface-material engaging surface of the encapsulantsolidifies with the thermal-interface-material engaging pattern thereon.

In another aspect, another exemplary method includes the step ofproviding an encapsulated electronic component mounted on a printedcircuit board. The encapsulated electronic component has anoutward-facing thermal-interface-material engaging pattern on anencapsulant thereof. A further step includes securing a heat sink to theencapsulated electronic component on the printed circuit board with athermal interface material intermediate the heat sink and thethermal-interface-material engaging pattern of the encapsulant such thatthe thermal-interface-material engaging pattern engages the thermalinterface material to resist lateral motion of the thermal interfacematerial.

In still another aspect, an exemplary assembly includes a printedcircuit board and an encapsulated electronic component mounted on theprinted circuit board. The encapsulated electronic component in turnincludes an encapsulant with an outward-facingthermal-interface-material engaging pattern thereon. Additional elementsinclude a heat sink and a thermal interface material intermediate theheat sink and the thermal-interface-material engaging pattern of theencapsulant. The heat sink is assembled to the printed circuit boardsuch that the thermal-interface-material engaging pattern engages thethermal interface material to resist lateral motion of the thermalinterface material.

As used herein, “facilitating” an action includes performing the action,making the action easier, helping to carry the action out, or causingthe action to be performed. Thus, by way of example and not limitation,instructions executing on one processor might facilitate an actioncarried out by instructions executing on a remote processor, by sendingappropriate data or commands to cause or aid the action to be performed.For the avoidance of doubt, where an actor facilitates an action byother than performing the action, the action is nevertheless performedby some entity or combination of entities.

Techniques of the present invention can provide substantial beneficialtechnical effects. For example, one or more embodiments reduce oreliminate undesirable displacement of thermal interface material withoutcostly additional processing and/or handling steps and/or debrisgeneration.

These and other features and advantages of the present invention willbecome apparent from the following detailed description of illustrativeembodiments thereof, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a relationship between a PCB assembly with encapsulatedelectronic components, heat sink, and TIM, according to the prior art;

FIG. 2 shows photographs of undesirable TIM displacement, according tothe prior art;

FIG. 3 shows variation in capacitive bondline with number of on-offcycles for the system of FIG. 2, according to the prior art;

FIG. 4 shows a relationship between a PCB assembly with encapsulatedelectronic components, heat sink, and TIM, according to an embodiment ofthe invention;

FIG. 5 shows details of a diamond pattern on encapsulation material ofthe assembly of FIG. 4, and also represents the negative of a portion ofa mold, according to an embodiment of the invention;

FIGS. 6 and 7 show a transfer molding process for producing theencapsulated electronic components of FIGS. 4 and 5, according to anembodiment of the invention;

FIG. 8 shows an embodiment wherein an entire surface of encapsulationmaterial is patterned, according to an embodiment of the invention; and

FIG. 9 shows an embodiment wherein a perimeter of a surface ofencapsulation material is patterned, according to an embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As noted above, electronic components are typically encapsulated bytransfer molding. For rapid and easy release from mold cavities, moldrelease agents are included in the formulation of transfer moldingcompounds. After the molding process, these release agents are presenton the surface of the molded components. It is very challenging forthermal interface materials (TIMs) to adhere to the resulting very lowsurface energy surfaces. In the past, the surface of the component hasbeen treated to provide higher energy surfaces for improved TIMadhesion. Such treatments include wiping the surface with isopropylalcohol, abrading the surface and then wiping with isopropyl alcohol, orexposing the surface to a plasma cleaning process or ultraviolet (UV)light and/or ozone. These are non-routine processes that are typicallyperformed at the card assembly factory and are significantly manual innature.

As also noted above, heat dissipation has become a major technicalchallenge for complex printed circuit board (PCB) assemblies. Ratherthan using individual heat sinks for every component, very large (inexcess of four square feet or 0.37 square meters), common heat spreadersare being used over an array of components and component types. FIG. 1shows a relationship between a PCB assembly with encapsulated electroniccomponents, heat sink, and TIM, according to the prior art. Note the PCB102, heat sink 104, thermal interface material 106, and encapsulatedelectronic components 108-1 and 108-2. Each component includes a leadframe 110-1, 110-2; a semiconductor die 112-1, 112-2; wires 114-1,114-2; encapsulant material 116-1, 116-2; and optionally an exposed pad118-1, 118-2.

Due to the mismatch in coefficient of thermal expansion between the heatspreader 104, which is typically aluminum (coefficient of thermalexpansion 23.6 ppm/C (parts per million per degree Celsius)) and PCBassembly (coefficient of thermal expansion ˜17 ppm/C for PCB 102) andthe large thermal mass of both the heat spreader and PCB assembly,significant relative movement occurs during temperature excursionsresulting from powering the circuitry on and off. The TIM 106 whichphysically bridges the gap between the heat spreader and PCB mustaccommodate this movement without breaking down. Because of the inherentlow surface energy of transfer molded component surfaces (i.e. surfaceof encapsulant 116-1, 116-2), the TIM can slide along these surfaceswhen it is sheared or compressed by the relative movement, and overtime, can move out of the gap. Shearing of course refers to lateralmovement of heat sink 104 with respect to PCB 102 while compressionrefers to heat sink 104 moving towards PCB 102. The result is increasingthermal resistance which leads to decreased reliability, and in a worstcase scenario, fire. Because of the size of the PCB assemblies, batchprocessing in plasma or UV and/or ozone is very costly. Abrasion andisopropyl wipe techniques are undesirable as well because of thehandling and debris generation.

A thermal interface material 106 between a heat spreader 104 andelectronic components (e.g. encapsulant 116-1, 116-2 thereof) undergoesmechanical deformation as a function of temperature cycles. This gapvariation can result in the thermal interface material being “pumped”out of the gap, which in turn results in an increase in the thermalresistance between the component and heat spreader.

FIG. 2 shows photographs of undesirable TIM displacement, according tothe prior art, while FIG. 3 shows variation in capacitive bondline withnumber of on-off cycles for the system of FIG. 2, according to the priorart. In particular, as shown in FIG. 2, a thermal interface material 206which was placed between a transfer molded component 209 and a heatspreader 208 showed movement after a few hundred thermal cycles. Theresults were observed by measuring the capacitive bondline, as in FIG.3, and by physical observation. Arrow 297 shows one edge of thedisplaced TIM which has been displaced from the corresponding area 295of the heat spreader 208, while arrow 299 shows how the TIM has beenpushed or extruded beyond the initial area of the heat spreader that wasmated to the transfer molded component.

FIG. 2 thus shows the TIM movement between a transfer molded component209 and heat spreader 208. The transfer molded component 209 has a pieceof copper tape on it so that capacitance measurement of the TIM in thegap could be carried out during thermal cycling. The aluminum heatspreader 208 had a surface roughness that was smoother than threemicrons and it can be seen that the TIM moved during thermal cycling asevidenced by both the changes in capacitive bond line in FIG. 3 as wellas the photo of the TIM movement in FIG. 2.

It was found by experiment that the TIM movement can be significantlyreduced by roughening the aluminum heat spreader to a roughness greaterthan three microns. To address the question about the impact of thecopper tape on the surface of the transfer molded component, anexperiment was conducted without the copper tape and with a smooth (lessthan three microns of roughness) aluminum heat spreader—the capacitivebond line could not be measured in this experiment because of no coppertape. However, after the cycling test, the heat spreader was separatedfrom the PCB and it was observed that there was evidence of the TIMmovement, just as when a smooth aluminum heat spreader was used and thecopper tape was present on the surface of the transfer molded component.It can be concluded that as long as one surface is roughened more thanthree microns, the TIM movement during a thermal cycle can besignificantly reduced. One or more embodiments address the transfermolder surface rather than the heat spreader surface.

FIG. 4 shows a relationship between a PCB assembly with encapsulatedelectronic components, heat sink, and TIM, according to an aspect of theinvention, wherein elements corresponding to those in FIG. 1 havereceived the same reference character incremented by three hundred. Notethe PCB 402, heat sink 404, thermal interface material 406, andencapsulated electronic components 408-1 and 408-2. Each componentincludes a lead frame 410-1, 410-2; a semiconductor die 412-1, 412-2;wires 414-1, 414-2; encapsulant material 416-1, 416-2; and optionally anexposed pad 418-1, 418-2.

One or more embodiments, such as the embodiment shown in FIG. 4,advantageously provide a low cost, manufacturable method of improvingthe adhesion of a thermal interface material between a transfer moldedelectronic component and a thermal heat spreader by modifying thecomponent surface during the molding process. In particular, one or moreembodiments pattern the mold (see discussion of FIG. 6 below) used inthe creation of a transfer molded electronic packaged component tocreate a surface roughness or distinct features (e.g. a diamond patternin top surface of encapsulant 416-1, 416-2 as seen in FIG. 5) whichimprove the adhesion characteristics of the thermal interface material406 and heat spreader 404. The incorporation of the pattern into themold provides a low cost method to create patterns which enhance theadhesion of the thermal interface material 406. Here, as seen in FIG. 5,a series of diamond-shaped “teeth” 589 are formed on the outward face ofthe encapsulant 416-1, 416-2 during the molding process, which teethengage or “bite” into the TIM 406 to reduce or eliminate relativemotion.

FIG. 5 also represents the negative of a portion of a mold, according toan embodiment of the invention, as per FIG. 6 discussed below.

Heat sink 404 can be liquid cooled, air cooled, or work via conduction,and can be made, for example, from aluminum, copper, or the like. PCB402 can be made of known materials such as a glass fiber reinforced(fiberglass) epoxy resin substrate with a copper foil bonded on to oneor both sides. These are non-limiting examples of materials.

One or more embodiments reduce or eliminate completely the need foradditional processes at the PCB assembly operation, inasmuch as patterns(e.g., of teeth 589) on the surface of a component in turn provide azero velocity boundary condition for a thermal interface material 406 onthe electronic overmolded surface 416-1, 416-2.

FIGS. 6 and 7 show a transfer molding process for producing theencapsulated electronic components of FIGS. 4 and 5, according to anembodiment of the invention. An upper mold portion 579 and lower moldportion 577 cooperatively define a cavity 575 which receives a componentto be encapsulated. The component includes a lead frame 610; asemiconductor die 612; wires 614; and optionally an exposed pad 618.Encapsulant material 616 in a reservoir 571 is provided to cavity 575through channel 569 and surrounds and encapsulates the component (FIG. 6shows encapsulant 616 just starting down channel 569 while FIG. 7 showsencapsulant 616 filling the cavity 575 and surrounding the component tobe encapsulated). Upper mold portion 579 is formed with a negative ofthe diamond pattern of FIG. 5 as seen at 573 and produces the positivepattern of FIG. 5 in the encapsulant 616 when same flows into the cavity575.

Only a single mold cavity and channel 569 are shown for simplicity butmultiple cavity molds can of course be employed for efficiency.

Thus, one or more embodiments provide a pattern on the roof of the moldcavity. In one or more embodiments, the pattern is imparted on the moldcavity surface and the transfer molding process does not need to bealtered in any other way. Regarding encapsulants, typical transfermolding compounds include a phenolic novolac epoxy resin and a phenolicnovolac hardener. A polyamine catalyst is used to speed up thecross-linking reaction. Silica filler is added, typically to 65 to 70%wt. The filler could be pretreated with either an epoxy or amino silaneadhesion promoter or the adhesion promoter could be added directly tothe formulation. The purpose of the adhesion promoter is to promotechemical bonding between the filler surface and the reacting epoxyresin. Additional additives in a typical transfer molding compoundformulation include flame retardants, mold release agents to facilitateremoval from the mold after transfer molding, carbon black as a pigment,and stress absorbing agents to manage thermal mechanical stresses in thetransfer molded electronic module.

Transfer molding is commonly done at 175 C, under 170 MPa of pressure in60 to 90 seconds—after being released from the mold cavity, parts arepost cured at 175 C for between four and eight hours; these parametersare exemplary and non-limiting.

In one or more embodiments, the patterned surface finish includesfinishes that have an average roughness (Ra) of 3.2 microns or more. Thesurface finish of the mold cavity could be imparted through beadblasting or machining, for example. In the case of bead blasting, eitherthe entire mold cavity surface can be treated, or a frame at theperimeter could be treated, serving as a blockade to movement out of thegap between a component surface and heat spreader. In the case ofmachining, specific patterns can be defined with a channel depth,cross-section and pitch. The channel patterns created can be providedwith an angled vertical wall, or draft angle of at least 1 degree, suchas an inverted triangle or trapezoidal cross-section to allow easyrelease after the transfer molding process.

FIG. 8 shows an embodiment where the entire surface of encapsulationmaterial 851 is patterned (corresponding to case where entire moldcavity surface is treated), while FIG. 9 shows an embodiment where onlythe perimeter of the surface of the encapsulation material 949 ispatterned, with an interior portion 947 not patterned (corresponding tocase where frame at the perimeter of the mold cavity surface istreated).

Non-limiting examples of TIM include known thermal greases, putties andgel materials that are soft and have a tendency to move or flow undercyclic stress.

Furthermore regarding channel patterns, hierarchal nested channelpatterns could be used in some instances as described in US PatentPublication 2006/0286712A1 of Thomas J. Brunschwiler et al., “Thermalinterface with a patterned structure,” the complete disclosure of whichis expressly incorporated herein by reference in its entirety for allpurposes. However, as stated above, the vertical walls of the channelsshould have a draft angle to allow easy release from the mold aftertransfer molding. In the case of machined patterns, channel depth canvary from 10 microns to a depth not to exceed the thickness of themolding compound above the wirebonds, which is typically 0.25 to 0.5 mmthick. To prevent the migration of a soft thermal interface material outof the gap between a component surface and a heat spreader, the depth inthe surface should not have to exceed 100 microns. More typically, adepth of 10 microns should be adequate. A minimum depth of three micronsis appropriate in one or more embodiments.

Given the discussion thus far, it will be appreciated that, in generalterms, an exemplary method includes the step of introducing into a moldcavity 575 an electronic component to be encapsulated (e.g., 610, 612,614, optionally 618). The mold cavity includes at least first and secondhalves 579, 577. At least one of the halves (in the example of FIG. 6,579) is formed with a negative 573 of a thermal-interface-materialengaging pattern thereon. A further step includes introducing into themold cavity 575 an encapsulating material 616 which encapsulates theelectronic component and engages the negative 573 of thethermal-interface-material engaging pattern.

An even further step includes allowing the encapsulating material tosolidify such that a thermal-interface-material engaging surface of theencapsulant solidifies with the thermal-interface-material engagingpattern thereon (as shown in FIGS. 4 and 5).

In some cases, further steps include mounting the electronic componenton a printed circuit board 402, and securing a heat sink 404 to theelectronic component on the printed circuit board with a thermalinterface material 406 intermediate the heat sink and thethermal-interface-material engaging pattern of the solidifiedencapsulating material, such that the thermal-interface-materialengaging pattern engages the thermal interface material 406 to resistlateral motion of the thermal interface material. Known techniques canbe used to secure the components together.

In some instances, in the securing step, the thermal interface materialincludes at least one of thermal grease, thermal putty, and thermal gel.In one or more embodiments, in the step of introducing the electroniccomponent into the mold cavity, the encapsulating material includes arelease agent. In at least some such cases, when the encapsulatingmaterial solidifies, the thermal-interface-material engaging surface hasa surface energy less than thirty five ergs per square centimeter.

In some cases, in the step of introducing the electronic component intothe mold cavity, the thermal-interface-material engaging pattern has anaverage roughness of at least three microns. As noted, non-limitingexamples of suitable patterns include a diamond pattern and/or a channelpattern.

Given the discussion thus far, it will be appreciated that, in generalterms, another exemplary method according to another aspect of theinvention includes the step of providing an encapsulated electroniccomponent (e.g. 410-1, 410-2; 412-1, 412-2, 414-1, 414-2; and optionally418-1, 418-2) mounted on a printed circuit board. The encapsulatedelectronic component has an outward-facing thermal-interface-materialengaging pattern on an encapsulant 416-1, 416-2 thereof (i.e., “teeth”on outer surface of the encapsulant). A further step includes securing aheat sink 404 to the encapsulated electronic component on the printedcircuit board 402 with a thermal interface material 406 intermediate theheat sink and the thermal-interface-material engaging pattern of theencapsulant 416-1, 416-2 such that the thermal-interface-materialengaging pattern engages the thermal interface material to resistlateral motion of the thermal interface material.

As discussed elsewhere, in the securing step, the thermal interfacematerial can include, for example, at least one of thermal grease,thermal putty, and thermal gel.

Further, as discussed elsewhere, in the providing step, theencapsulating material can include a release agent. Thethermal-interface-material engaging pattern may have a surface energyless than thirty five ergs per square centimeter.

The thermal-interface-material engaging pattern has, in one or moreembodiments, an average roughness of at least three microns, and asnoted, can include, for example, a diamond pattern or a channel pattern.

In any of the methods disclosed herein, the method steps canadvantageously be carried out without wiping thethermal-interface-material engaging pattern (surface) with alcohol,without abrading the thermal-interface-material engaging pattern(surface), without exposing the thermal-interface-material engagingpattern (surface) to a plasma cleaning process, without exposing thethermal-interface-material engaging pattern (surface) to ultravioletlight, and without exposing the thermal-interface-material engagingpattern (surface) to ozone.

Furthermore, given the discussion thus far, it will be appreciated that,in general terms, an exemplary assembly according to another aspect ofthe invention includes a printed circuit board 402 and an encapsulatedelectronic component (e.g. 410-1, 410-2; 412-1, 412-2, 414-1, 414-2; andoptionally 418-1, 418-2) mounted on the printed circuit board. Theencapsulated electronic component in turn includes an encapsulant 416-1,416-2 with an outward-facing thermal-interface-material engaging patternthereon. Also included are a heat sink 404 and a thermal interfacematerial 406 intermediate the heat sink and thethermal-interface-material engaging pattern of the encapsulant. The heatsink is assembled to the printed circuit board such that thethermal-interface-material engaging pattern engages the thermalinterface material to resist lateral motion of the thermal interfacematerial.

The thermal interface material can be, for example, thermal grease,thermal putty, and/or thermal gel. In one or more embodiments, theencapsulant includes a release agent and the outward-facingthermal-interface-material engaging pattern has a surface energy lessthan thirty five ergs per square centimeter.

As noted, the outward-facing thermal-interface-material engaging patterncan have an average roughness of at least three microns. Theoutward-facing thermal-interface-material engaging pattern can havedraft angle of at least one degree from vertical. The outward-facingthermal-interface-material engaging pattern can be, for example, adiamond pattern or a channel pattern.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method comprising: providing an encapsulatedelectronic component mounted on a printed circuit board, saidencapsulated electronic component having an outward-facingthermal-interface-material engaging pattern on an encapsulant thereof;and securing a heat sink to said encapsulated electronic component onsaid printed circuit board with a thermal interface materialintermediate said heat sink and said thermal-interface-material engagingpattern of said encapsulant such that said thermal-interface-materialengaging pattern engages said thermal interface material to resistlateral motion of said thermal interface material.
 2. The method ofclaim 1, wherein, in said securing step, said thermal interface materialcomprises at least one of thermal grease, thermal putty, and thermalgel.
 3. The method of claim 1, wherein, in said providing step, saidencapsulating material comprises a release agent and saidthermal-interface-material engaging pattern has a surface energy lessthan thirty five ergs per square centimeter.
 4. The method of claim 1,wherein said steps are carried out without wiping thethermal-interface-material engaging pattern with alcohol, withoutabrading the thermal-interface-material engaging pattern, withoutexposing the thermal-interface-material engaging pattern to a plasmacleaning process, without exposing the thermal-interface-materialengaging pattern to ultraviolet light, and without exposing thethermal-interface-material engaging pattern to ozone.
 5. The method ofclaim 1, wherein, in said providing step, saidthermal-interface-material engaging pattern has an average roughness ofat least three microns.
 6. The method of claim 1, wherein, in saidproviding step, said thermal-interface-material engaging patterncomprises at least one of a diamond pattern and a channel pattern.
 7. Anassembly comprising: a printed circuit board; an encapsulated electroniccomponent mounted on said printed circuit board, said encapsulatedelectronic component in turn comprising an encapsulant with anoutward-facing thermal-interface-material engaging pattern thereon; aheat sink; and a thermal interface material intermediate said heat sinkand said thermal-interface-material engaging pattern of saidencapsulant; wherein said heat sink is assembled to said printed circuitboard such that said thermal-interface-material engaging pattern engagessaid thermal interface material to resist lateral motion of said thermalinterface material.
 8. The assembly of claim 7, wherein said thermalinterface material comprises at least one of thermal grease, thermalputty, and thermal gel.
 9. The assembly of claim 7, wherein saidencapsulant comprises a release agent and said outward-facingthermal-interface-material engaging pattern has a surface energy lessthan thirty five ergs per square centimeter.
 10. The assembly of claim7, wherein said outward-facing thermal-interface-material engagingpattern has an average roughness of at least three microns.
 11. Theassembly of claim 7, wherein said thermal-interface-material engagingpattern has a draft angle of at least one degree from vertical.
 12. Theassembly of claim 7, wherein said thermal-interface-material engagingpattern comprises at least one of a diamond pattern and a channelpattern.